The pathophysiology of glaucoma, one of the most common causes of visual impairment, is not clearly understood. Several recent reports have suggested that abnormalities of the circulation of the optic nerve head may increase the susceptibility to glaucomatous damage. The main goal of this grant is to investigate whether optic nerve head circulatory abnormalities are present in patients with primary open- angle glaucoma (POAG), normal-tension glaucoma (NTG) and ocular hypertension (OHT). To achieve this goal we will use laser Doppler flowmetry, a non-invasive technique that provides relative measurements of blood velocity, blood volume and blood flow, to study optic nerve head hemodynamics. This grant has three specific aims which will test the following hypotheses: 1) Baseline optic nerve head blood flow in patients with POAG, NTG and OHT is different from normal. In POAG patients with asymmetric disease between eyes and no visual field in the fellow eye, abnormal blood flow is present in the eye with normal visual field. 2) Vascular autoregulatory responses to acute increases in IOP in these patients are different from normal. 3) Abnormalities of baseline optic nerve head blood flow and its autoregulation in POAG patients with asymmetric visual field defects between superior and inferior hemifield correlate with areas of glaucomatous damage. The results obtained from these studies will help us assess whether optic nerve head circulatory abnormalities are associated with the development of glaucomatous pathology. If hemodynamic abnormalities are present, then attention can be focused on altering the optic nerve circulation as a means of treating glaucoma.